A telecommunication network power system usually includes an ac-dc stage converting the power from the ac utility line to a 48V dc distribution bus and a dc/dc stage converting the 48V dc distribution bus to a plurality of voltage levels for all types of telecommunication loads. A conventional ac-dc stage may comprise a variety of EMI filters, a bridge rectifier formed by four diodes, a power factor correction circuit and an isolated dc/dc power converter. The dc/dc stage may comprise a plurality of isolated dc/dc converters. Isolated dc/dc converters can be implemented by using different power topologies, such as LLC resonant converters, flyback converters, forward converters, half bridge converters, full bridge converters and the like.
Active clamp forward converters are widely adopted for small to medium power level isolated power converters in the telecommunications and data communications industries. During a power up of an active clamp forward converter, the secondary side of the active clamp forward converter may operate a diode rectifier mode first when a pre-biased voltage is applied to the output of the active clamp forward converter during the power up. After the output of the active clamp forward converter is more than the pre-biased voltage, the active clamp forward converter may move from diode rectification to synchronous rectification. Such a shift may improve the efficiency of the active clamp forward converter.